Astronomy’s Bat Signal

When a rare cosmic neutrino hit the South Pole, alarms around the world went off.

The IceCube Neutrino Observatory at the South Pole sits atop under-ice detectors (artist impression) that catch the blue glow created by a cosmic neutrino. (IceCube/NSF)
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A blue light flashed under the ice at the South Pole, and in 43 seconds, the world knew. A neutrino—a strange, almost massless particle created in a cataclysm in space—had hit the ice and created a subatomic particle that emitted the glow. Because neutrinos interact with matter only at the subatomic scale, the only way scientists can find them is by looking for radiation from a secondary particle created in a collision. A program that filters through thousands of these detections a second, looking for the unusually high energy that indicates a cosmic neutrino, recognized this glow and pinged a powerful Iridium satellite modem, set up specially to blast alerts from the bottom of the world. And on a Friday afternoon last September, Erik Blaufuss, in Maryland, got a text alert from the South Pole.

Blaufuss is the alerts coordinator for the IceCube Neutrino Observatory, which recently announced the first detection of a cosmic neutrino coming from an identified event in space. The facility was built by the National Science Foundation in 2011, and by 2013, the team had detected its first neutrino from space, but confirming that discovery and those that followed came slowly, and astronomers couldn’t tell much about the particles. They would broadcast their data, but “if these events [in space] were in any way transient, you were just too late,” says Blaufuss. So he led the charge to set up IceCube’s alert system, which uses a network created by gamma ray scientists and now co-opted by astronomers in other fields.

A few hours after Blaufuss got his text message, Yasuyuki Tanaka, in Japan, was monitoring alerts for the Fermi Gamma-ray Space Telescope team. He saw IceCube’s alert, indicating the patch in the sky from which the neutrino came, and began looking through the Fermi’s observations. There it was: a blazar, a galaxy with a black hole at its center that shoots out jets of high-energy particles. The black hole was in the midst of a violent flare. Tanaka and David Thompson, Fermi’s deputy project scientist, sent out their own message saying they may have found the neutrino’s birthplace, and soon other observatories—radio, optical, all across the spectrum—were chiming in. “And that’s how we wound up with this paper in Science that had one thousand authors,” says Thompson.

This is the latest example of what’s being called multi-messenger astronomy—the coordinated study of light, gravitational waves, cosmic rays, and neutrinos. Another example came last fall when the detection of a gravitational wave was followed up by optical observatories that located the merging neutron stars that created the ripple in space time. With astronomers from so many fields now working together, understanding the mysteries of neutrinos seems imminent.

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